A human being has the ability to know the location of a sound source based on a plurality of factors such as interaural intensity difference (IID), interaural time difference (ITD), and changes in frequency characteristics by using two ears. Generally, music recorded on a compact disc and so on has been mixed down using a music mixing system by adjusting the sound volumes and localization or panpots of a plurality of sound sources (musical instruments etc.) so that the respective sound sources are well separated for ready listening. Accordingly, when the mixed-down music is reproduced, the sound volumes and localization of the plurality of sound sources have already been fixed. It is impossible for users to adjust them even when they want to increase the sound volume of a specific sound source, for example, when users want to increase the sound volume of the first violin in a piece of classical music without changing the sound volume of the music piece itself.
If users could manipulate the music mixing system by hand while listening to the music, it would be possible to change the sound volume and localization or panpot of the specific sound source. Actually, however, manipulation of the music mixing system is too complicated for amateur users to operate intuitively. Incidentally, a mixing apparatus with a recording/reproducing function disclosed in Japanese Patent No. 3633459 (refer to Patent Reference 1) is complicated in operation, and it is not easy for ordinary users (listeners) to increase the sound volume of a particular part they want to hear, or decrease the sound volume of a particular part they do not want to hear when listening to the music.
For example, it is difficult to quickly perform a series of adjusting operations to increase the sound volume of a violin, to set the panpot at the center of two speakers (setting the sound volume ratio of the violin output from the left/right speakers to 1/1), and adjusting the localization or panpots of the other musical instruments by slightly decreasing the sound volumes thereof when the violin solo part has started.
By a method of controlling a system for producing a realistic musical sensation disclosed in Japanese Patent Application Publication No. 2000-69600 (Patent Reference 2), it may be possible to arrange sound sources in a three-dimensional space and to change the locations of the sound sources and listeners. However, by this method, it is difficult to for a listener to arrange near the listener the a particular part of the music that the listener wants to hear and to increase the sound volume of that part, or to arrange apart from the listener a particular part that the listener does not want to hear and to decrease the sound volume of that part while the listener is listening to the music.
A conventional product equipped with a gyro-sensor (acceleration sensor) on the headphone is substantially different from the present invention in which mixing is performed by changing the sound volume and localization or panpot of each sound source using information acquired from sensors. This conventional product is intended to prevent a disorder of the sound field due to the listener's head movement (that is, when the listener moved his/her head in a certain direction, he/she feels like the sound source accordingly moving in the same direction) by finely adjusting the sound direction in accordance with the inclination of the listener's head. Japanese Patent Application Publication No. 02-25900 (Patent Reference 3) discloses a technique to enhance a realistic musical sensation by mounting a vibration gyroscope onto a headphone a listener wears on the head so as to detect movements of the listener's head and to adjust the sound volume on either side of the headphone in accordance with the detected movements of the listener's head. In this manner, sound sources may be fixedly located within a space even when the listener's head moves, thus enhancing the realistic musical sensation. Japanese Patent Application Publication No. 08-9490 (Patent Reference 4) discloses a technique to detect a turning angle of the listener's head by using a microphone mounted onto the headphone. Further, Japanese Patent Application Publication No. 09-205700 (Patent Reference 5) discloses a technique to detect in which direction the listener's head equipped with the headphone is facing. Furthermore, Japanese Patent Application Publication No. 08-237790 (Patent Reference 6) discloses a technique to detect information on not only a turn of a listener's head but also the facing direction and location of the listener.
With existing amplifiers which are capable of separately adjusting an amplification factor for either side speaker, and existing vocal canceling machines, namely, equipment capable of removing a sound localized at the center, it may be possible to partially change the sound volumes and panpots of respective parts of the mixed-down music, but such apparatuses are not practically usable in adjusting the sound volume or localization of a specific sound source.    Patent Reference 1: Japanese Patent No. 3633459.    Patent Reference 2: Japanese Patent Application Publication No. 2000-69600    Patent Reference 3: Japanese Patent Application Publication No. 02-25900    Patent Reference 4: Japanese Patent Application Publication No. 08-9490    Patent Reference 5: Japanese Patent Application Publication No. 09-205700    Patent Reference 6: Japanese Patent Application Publication No. 08-237790